The population biology of bacteria - their viruses and plasmids - is being investigated at both a theoretical and experimental level. The former involves the development and analysis of the properties of mathematical models of these systems and the latter experiments with strains of E. coli in serial transfer and continuous culture. Currently this work is being done with three conjugative plasmids, F-lac pro, Rl (cm, km, am) and Rl-drd-19 (cm, kmam), three nonconjugative plasmids PSC101 (tc), PCRl (km) and CO1E3, and two phage and lambda and MS2. The primary concern of these studies is to ascertain the conditions for the establishment and maintenance of these extrachromosomal elements and viruses in bacterial population and the mechanisms and effects of selection in these systems. To achieve these goals extensive consideration is also being given to the kinetics of transfer or mobilization of these plasmids and the kinetics of infection of MS2 in an E. coli host. Some work is also being done to study natural population variation and the process of selection at the phoA (alkaline phosphatase) and lacZ (beta-galatosidase) loci of E. coli. The primary concern of these molecular evolutionary studies is to ascertain the relationship between the catalytic efficiences of these enzymes and the fitness of the alleles coding for their synthesis.